PROJECT SUMMARY Virus-specific CD8+ T cells are essential for immune defenses against many viruses. However, during chronic infections in mice and humans, CD8+ T cells undergo excessive apoptosis or become functionally inactive and unable to resolve infection. T cells fail to eliminate infection due to their sustained expression of inhibitory receptors that actively suppress their antiviral activity. Despite evidence that therapeutic interference with inhibitory receptor signaling can allow some T cells to recover function and reduce viral loads, there continues to be a need for improved methods to prevent T cell exhaustion from occuring or to reverse T cell exhaustion once it sets in. It is now understood that virus-specific T cell senescence and T cell exhaustion are programmed epigenetically and guided by specific sets of transcription factors. Distinct epigenetic processes, including changes in the methylation status of histone-3 lysine-27 (H3K27) in CD8+ T cells, contribute to the formation of memory T cells, as well as functionally inactive subsets during chronic infection. The role of specific enzymes in this process is poorly understood. We recently identified a critical link between CD8+ T cell expression of UTX, an H3K27 demethylase, and impaired CD8+ T cell responses to chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. Virus-specific CD8+ T cells lacking UTX showed improved accumulation and maintenace over time, reduced expression of the inhibitory receptors, and were resistant to apoptosis. Our data suggest that UTX restricts virus-specific CD8+ T cell responses by increasing T cell expression of inhibitory receptors, perhaps converting cells with memory potential into functionally exhausted or senescent cells. Our central hypothesis is that UTX controls T cell differentiation through a mixture of demethylase-dependent and demethylase-independent mechanisms that promote gene expression, including at inhibitory receptor loci. In Aim1 we determine the temporal relationship between UTX and CD8+ T cell differentiation into functional or dysfunctional T cell subsets. In Aim2, we investigate the role of UTX-mediated demethylase activity in regulating gene expression and T cell effector functions and maintenance during infection. In Aim3, we explore the role of UTX interacting partners, including T cell-relevant transcription factors and an H3K4 methyltransferase, in guiding T cell differentiation and antiviral functions. Information from this project will be useful for comprehending how epigenetic changes due to histone methylation guide CD8+ T cell exhaustion. Long-term, our research may implicate the use of pharmacologic inhibitors of UTX to improve CD8+ T cell-mediated immune defenses.